A method is known to purify harmful substances such as hydrocarbons (HC), carbon monoxide (CO), sulfur (S) and nitrogen oxides (NOx) contained in the exhaust gas discharged from a vehicle engine by using a metal catalyst. In such purification methods, however, processing ability is low when the catalyst is not fully activated, e.g. immediately after engine startup.
A method is also known to reduce the production of NOx in an engine by recirculating part of the exhaust gas into the engine intake air. In order to achieve considerable reduction of NOx, the exhaust recirculation amount must be large, for purifying harmful substances in the exhaust gas of an engine.